The present invention relates to an image input device and, more particularly, to an image input device for optically sensing fingerprints and other patterns to convert them to electrical signals.
Implementations available today for identifying humans and distinguishing them from one another include a system which relies on fingerprints. By virtue of the idosyncrasy and invariability of fingerprints and aided by pattern recognition technology, such an identification system compares characteristics, or minutiae, of fingerprints with those of registered fingerprints for similarities.
In a device based on the above-mentioned identification principle, a finger is laid on a predetermined contact surface of a glass prism and illuminated by light from behind the contact surface, so that the light reflected from the contact surface is converted to an electrical signal representative of a pattern peculiar to the fingerprint. This kind of pattern collection utilizes optical boundary changes of glass which occur at the contact surface of the prism due to the fingerprint pattern. Changes in the intensity of reflected light due to the optical boundary changes are picked up by an industrial television (ITV) camera or like imaging apparatus to provide a photoelectrically converted image of the fingerprint pattern. The described type of apparatus is disclosed in Japanese Patent Disclosure Nos. 54-69300 and 54-85600, for example.
The prism in any of the prior art apparatuses mentioned above has an incidence surface to which light from a light source is incident, a reflective surface for reflecting the incident light and on which a finger is laid, and a wayout surface through which light from the reflective surface emerges to the outside. In this condition, the lengthwise direction of the pattern of the fingerprint (identical with the lengthwise direction of the finger) is positioned substantially perpendicular to the ridge which is defined by the incidence and wayout surfaces of the prism. This brings about a problem that due to the triangular cross-section of the prism the image pattern coming out of the wayout surface to be picked up by the camera is blurred at both ends portions with respect to the lengthwise direction.